FiberLean Technologies have developed a unique process to convert a mixture of paper pulp and particulate materials into a microfibrillated cellulose (MFC)/mineral composite. The use of this material to enhance the strength, optical, and surface properties of paper or paperboard, and to increase its mineral filler loading, is well established. To date, the company has built four satellite plants at paper mills, making the composite on-site for both graphic paper and packaging grades, including those certified for food contact.
In all of these commercial applications, MFC is added directly to the paper furnish, and it greatly reduces the air permeability (Bendtsen porosity) of the sheet. In most cases, it also improves the surface smoothness. The reduction in porosity in particular improves coating holdout; this has been shown to provide a key economic benefit in graphic paper grades, as it has enabled the papermaker to lower coatweight and reduce the use of polymer latex binder whilst still maintaining all of the important coating properties.
The use of water-based barrier coatings in place of laminated polymer layers has been a goal of the packaging industry for several years. In order to achieve the necessary barrier properties at an economically viable coatweight, holdout of the coating and minimization of the number of defects and imperfections in it are critical, so the benefits of adding MFC to the sheet are evident.
An alternative way of adding MFC to paper is to apply it as a coating. However, since MFC suspensions have very high viscosity even at solids contents of a few percent, only very low coatweights can be applied with conventional coating techniques such as the metered size press. We have been developing a method to apply a coating of the FiberLean MFC/mineral composite immediately after the wet line of a paper machine in order to be able to generate sufficient coatweight. Even though the paper is still 90% water at this point, the MFC prevents the coating from penetrating the sheet, and by applying it in this way, existing vacuum and pressing elements on the machine can remove the added water. We have demonstrated that this can be done at realistic speeds in trials on fast pilot machines.
Coatings of MFC have better moisture barrier properties than paper, but these are still insufficient for most applications, particularly in humid conditions. However, the oil and grease resistance of MFC-based coatings can be excellent. Where moisture barrier is needed, an MFC coating applied at the wet end can form an excellent precoat for a conventional barrier coating, which we demonstrate with a combination of laboratory and pilot-scale data.